Double-acting hot gas engines generally comprises three or more working gas charges each of which are cyclically compressed at low temperature, heated, expanded at high temperature and cooled. The work performed during a working cycle is almost proportional to the amount of working gas in the charge.
Therefore, it is common practise to vary the amount of working gas in each working gas charge in order to govern the power output--e.g. in case the engine speed and the engine temperatures should be kept nearly constant.
The U.S. Pat. No. 3,927,529 shows a power control system of this known type. In order to avoid interconnections between the various working gas charges of the engine while being able to perform a simultaneous gas supply or gas removal by means common to all working gas charges, the gas supply system should comprise two check valves at each connection to a part of the working gas charge limiting device--e.g. the cold gas connection duct between the low temperature part of the cylinder and an adjacent cooler.
It is also known from the U.S. Pat. No. 3,927,529 to provide each piston with two piston rings each of which allowing flow of gas in one direction only across the piston ring. A minimum working cycle gas pressure will prevail in the space between the two piston rings in case both piston rings allow gas flow only away from said space. The two piston rings act as check valves and the purpose is to maintain an equal mean pressure in all working gas cycles of the engine.
The U.S. Pat. No. 4,417,444 shows a double-acting hot gas engine having axially spaced piston rings at each piston. The axial distance is greater than the piston stroke and a connection is established between the space between the piston rings and a source of gas having minimum or maximum pressure. Again the purpose is to maintain equal pressures in working gas charges.